Inactivation of tumor suppressor genes leads to unregulated cell proliferation and is a cause of tumorigenesis. In many tumors, the tumor suppressors, p53 or Rb (retinoblastoma) are inactivated. This can occur either by mutations within these genes, or by overexpression of the mdm2 gene. The mdm2 protein physically associates with both p53 and Rb, inhibiting their function. The levels of mdm2 are maintained through a feedback loop mechanism with p53. Overexpression of mdm2 effectively inactivates p53 and promotes cell proliferation.
The role of p53 in apoptosis and tumorigenesis is well-known in the art (see, in general, Canman, C. E. and Kastan, M. B., Adv. Pharmacol., 1997, 41, 429-460). Mdm2 has been shown to regulate p53's apoptotic functions (Chen, J., et al., Mol. Cell Biol., 1996, 16, 2445-2452; Haupt, Y., et al., EMBO J., 1996, 15, 1596-1606). Overexpression of mdm2 protects tumor cells from p53-mediated apoptosis. Thus, mdm2 is an attractive target for cancers associated with altered p53 expression.
Amplification of the mdm2 gene is found in many human cancers, including soft tissue sarcomas, astrocytomas, glioblastomas, breast cancers and non-small cell lung carcinomas. In many blood cancers, overexpression of mdm2 can occur with a normal copy number. This has been attributed to enhanced translation of mdm2 mRNA, which is thought to be related to a distinct 5'-untranslated region (5'-UTR) which causes the transcript to be translated more efficiently than the normal mdm2 transcript. Landers et al., Cancer Res. 57, 3562, (1997).
Several approaches have been used to disrupt the interaction between p53 and mdm2. Small peptide inhibitors, screened from a phage display library, have been shown in ELISA assays to disrupt this interaction [Bottger et al., J. Mol. Biol., 269, 744 (1997)]. Microinjection of an anti-mdm2 antibody targeted to the p53-binding domain of mdm2 increased p53-dependent transcription [Blaydes et al., Oncogene, 14, 1859 (1997)].
A vector-based antisense approach has been used to study the function of mdm2. Using a rhabdomyosarcoma model, Fiddler et al. [Mol. Cell Biol., 16, 5048 (1996)] demonstrated that amplified mdm2 inhibits the ability of MyoD to function as a transcription factor. Furthermore, expression of full-length antisense mdm2 from a cytomegalovirus promoter-containing vector restores muscle-specific gene expression.
Antisense oligonucleotides have also been useful in understanding the role of mdm2 in regulation of p53. An antisense oligonucleotide directed to the mdm2 start codon allowed cisplatin-induced p53-mediated apoptosis to occur in a cell line overexpressing mdm2 [Kondo et al., Oncogene, 10, 2001 (1995)]. The same oligonucleotide was found to inhibit the expression of P-glycoprotein [Kondo et al., Br. J. Cancer, 74, 1263 (1996)]. P-glycoprotein was shown to be induced by mdm2. Teoh et al [Blood, 90, 1982 (1997)] demonstrated that treatment with an identical mdm2 antisense oligonucleotide or a shorter version within the same region in a tumor cell line decreased DNA synthesis and cell viability and triggered apoptosis.
Chen et al. [Proc. Natl. Acad. Sci. USA, 95, 195 (1998); WO 99/10486] disclose antisense oligonucleotides targeted to the coding region of mdm2. A reduction in mdm2 RNA and protein levels was seen, and transcriptional activity from a p53-responsive promoter was increased after oligonucleotide treatment of JAR (choriocarcinoma) or SJSA (osteosarcoma) cells.
WO 93/20238 and WO 97/09343 disclose, in general, the use of antisense constructs, antisense oligonucleotides, ribozymes and triplex-forming oligonucleotides to detect or to inhibit expression of mdm2. EP 635068B1, issued Nov. 5, 1997, describes methods of treating in vitro neoplastic cells with an inhibitor of mdm2, and inhibitory compounds, including antisense oligonucleotides and triple-strand forming oligonucleotides.
There remains a long-felt need for improved compositions and methods for inhibiting mdm2 gene expression.